Fragile X syndrome (FXS) is the most common form of inherited cognitive impairment, and children with this disorder often have additional behavioral and neurological problems, including increased anxiety, autistic tendencies, hyperactivity and epilepsy. Since the GABA system plays an important role in regulation of the neural systems involved in these behavioral phenotypes, GABA system deficits could be present in FXS. While alterations in GABAA receptors (GABAARs) have been identified, knowledge of the regional and cellular localization of changes in GABAAR subunits remains very limited. Thus, the broad goal of this project is to test the hypothesis that expression and localization of specific subunits of the GABAAR are altered during postnatal development in a mouse model of FXS and that these changes are associated with functional deficits, including increased neuronal excitability and altered anxiety- related behavior. Specific Aim 1 will identify changes in the 12 subunit of the GABAAR in Fmr1 knockout mice that lack the Fragile X mental retardation protein (FMRP), using immunohistochemical methods. Importantly, the patterns of expression of the 12 subunit will be followed throughout early postnatal development in wild-type and Fmr1 knockout mice in order to identify early changes that could be associated with loss of FMRP function and precede potential compensatory changes that may become apparent later in life. Specific Aim 2 will identify functional deficits that could be associated with altered expression of the 12 subunit in Fmr1 knockout mice. In vitro electrophysiological studies will be used to assess GABAergic function at the axon initial segment of dentate granule cells where the 12 subunit is normally prominent and where a loss of 12 subunit-containing GABAARs could influence axon potential generation and increase granule cell excitability. Behavioral tests will be used to evaluate the anxiolytic and sedative effects of a classical benzodiazepine that could be altered in response to GABAAR subunit changes in Fmr1-deficient mice. Specific Aim 3 will identify changes in the expression and localization of the 4 subunit of the GABAAR in the Fmr1 knockout mouse and determine if such changes are reflected in alterations in tonic inhibition and its modulation by neurosteroids in the dentate gyrus. Specific Aim 4 will determine if GABAAR-related pharmacological treatment will ameliorate some of the behavioral changes and GABAAR subunit deficits in Fmr1 knockout mice. These studies will provide unique information about GABAAR subunit alterations during postnatal development in a mouse model of FXS and could provide a framework for new GABAAR subunit-targeted treatments for this disorder. PUBLIC HEALTH RELEVANCE: The proposed research is relevant to public health because it will provide new information about changes in GABA receptors during development in a model of Fragile X syndrome, the most common form of inherited cognitive impairment in humans. The project is relevant to NIH's mission because it could provide new views of the basic mechanisms underlying Fragile X syndrome and related disorders and suggest novel treatment approaches that could include targeting specific subunits of GABA receptors.